Triceps brachii m

The triceps brachii m. is the only muscle in the posterior compartment of the arm, and functions to extend the shoulder and the elbow joint.  The three heads are lateral, long, and medial.  Both the lateral and the medial head originate from the humerus, while the long head originates from the infraglenoid tubercle of the scapula.  The three heads have a common tendon of insertion into the olecranon process of the ulna, and all three are innervated by the radial n.

Yellow - lateral, Red - long, Green - medial